Expression of transgenes in plants is strongly affected by various external and internal factors resulting in a variable and unpredictable level of transgene expression. Often a high number of transformants have to be produced and analyzed in order to identify lines with desirable expression strength. As transformation and screening for lines with desirable expression strength is costly and labor intensive there is a need for high expression of one or more transgenes in a plant. This problem is especially pronounced, when several genes have to be coordinately expressed in a transgenic plant in order to achieve a specific effect as a plant has to be identified in which each and every gene is strongly expressed.
For example, expression of a transgene can vary significantly, depending on construct design and positional effects of the T-DNA insertion locus in individual transformation events.
Strong promoters can partially overcome these challenges. However, availability of suitable promoters showing strong expression with the desired specificity is often limited. In order to ensure availability of sufficient promoters with desired expression specificity, the identification and characterization of additional promoters can help to close this gap. However, natural availability of promoters of the respective specificity and strength and the time consuming characterization of promoter candidates impedes the identification of suitable new promoters.
In order to overcome these challenges, diverse genetic elements and/or motifs have been shown to positively affect gene expression. Among these, some introns have been recognized as genetic elements with a strong potential for improving gene expression. Although the mechanism is largely unknown, it has been shown that some introns positively affect the steady state amount of mature mRNA, possibly by enhanced transcriptional activity, improved mRNA maturation, enhanced nuclear mRNA export and/or improved translation initiation (e.g. Huang and Gorman, 1990; Le Hir et al., 2003; Nott et al., 2004). Since only selected introns were shown to increase expression, splicing as such is likely not accountable for the observed effects.
The increase of gene expression observed upon functionally linking introns to promoters is called intron mediated enhancement (IME) of gene expression and has been shown in various monocotyledonous (e.g. Callis et al., 1987; Vasil et al., 1989; Bruce et al., 1990; Lu et al., 2008) and dicotyledonous plants (e.g. Chung et al., 2006; Kim et al., 2006; Rose et al., 2008). In this respect, the position of the intron in relation to the translational start site (ATG) was shown to be crucial for intron mediated enhancement of gene expression (Rose et al., 2004).
Next to their potential for enhancing gene expression, few introns were shown to also affect the tissue specificity in their native nucleotide environment in plants. Reporter gene expression was found to be dependent on the presence of genomic regions containing up to two introns (Sieburth et al., 1997; Wang et al., 2004). 5′ UTR introns have also been reported to be of importance for proper functionality of promoter elements, likely due to tissue specific gene control elements residing in the introns (Fu et al., 1995a; Fu et al., 1995b; Vitale et al., 2003; Kim et al., 2006). However, these studies also show that combination of introns with heterologous promoters can have strong negative impacts on strength and/or specificity of gene expression (Vitale et al., 2003; Kim et al., 2006, WO2006/003186, WO2007/098042). For example the strong constitutive Cauliflower Mosaic Virus CaMV35S promoter is negatively affected through combination with the sesame SeFAD2 5′UTR intron (Kim et al., 2006). In contrast to these observations, some documents show enhanced expression of a nucleic acid by IME without affecting the tissue specificity of the respective promoter (Schünmann et al., 2004). Introns or NEENAs that enhance seed-specific and/or seed-preferential expression when functionally linked to a heterologous promoter have not been shown in the art.
In the present application further nucleic acid molecules are described that enhance the expression of said promoters without affecting their specificity upon functionally linkage to seed-specific and/or seed-preferential promoters. These nucleic acid molecules are in the present application described as “nucleic acid expression enhancing nucleic acids” (NEENA). Introns have the intrinsic feature to be spliced out of the respective pre-mRNA. In contrast to that the nucleic acids presented in the application at hand, do not necessarily have to be included in the mRNA or, if present in the mRNA, have not necessarily to be spliced out of the mRNA in order to enhance the expression derived from the promoter the NEENAs are functionally linked to.